Sharks rely on their sense of smell to help chart a path through the ever-shifting waters of the deep seas, according to a new study. (Read more about the secrets of animal navigation.)
Many sharks make epic journeys: Great white sharks routinely swim from Hawaii to California, and salmon sharks migrate between the Alaska coast and the subtropical Pacific.
Scientists have hypothesized that the animals navigate by monitoring odor cues or the Earth’s magnetic field, but no one knew for sure.
In new experiments near San Diego (map), scientists ferried wild leopard sharks about 6 miles (10 kilometers) away from their preferred hangout, fitted them with tracking devices, and stuffed some of the animals' nostrils with cotton balls.
Just 30 minutes after being released facing the wrong way, sharks with full use of their sniffers “made a corrective U-turn and then beelined it back to shore,” says study leader Andrew Nosal, a postdoctoral researcher at the Scripps Institution of Oceanography and the Birch Aquarium in La Jolla, California. (See amazing shark pictures.)
Sharks with clogged noses, meanwhile, “appeared lost," meandering aimlessly and swimming more slowly than those that could smell freely, Nosal says.
Sniffing Out Home
To test sharks' sense of direction, Nosal and colleagues captured several dozen leopard sharks, a small species found along the coast from Washington State to northern Mexico.
The hooked fish, all adult females, averaged only 5 feet (1.5 meters) long, but “put up a really good fight,” Nosal says.
After blocking some of the animals' nostrils, the scientists then took the animals on a cruise to deeper waters before slipping them, safe and unharmed, back into the sea.
Even the sharks with plugged nostrils made it partway back to shore before their tracking devices fell off, the scientists report in the January 6 issue of the journal PLOS ONE.
But those with unplugged nostrils “took very straight paths" toward home. (See shark pictures by National Geographic readers.)
Nosal hypothesizes the sharks likely sniffed out chemical molecules found in higher and higher doses nearer to land.
Only the First Step
Other scientists, however, remain unconvinced.
Maybe the animals with plugged nostrils “were confounded by the fact that they had something stuffed in their nose,” says Kim Holland, a marine biologist at the University of Hawaii at Manoa.
It’s also unlikely the animals were following an odor that grew stronger closer to land, adds Jayne Gardiner, a sensory biologist at the New College of Florida.
Perhaps the sharks were sniffing something from the direction of the land that drew their attention and then followed other cues, such as water temperature or light levels, to their usual stomping grounds, she says. (Also see "5 Amazing Animal Navigators.")
Animals that couldn’t smell a thing still turned toward the beach, which “suggests something else is really guiding them,” she says.
Study leader Nosal responds that sharks with cotton-stuffed nostrils willingly eat, suggesting that a congested nose doesn’t upset them much.
He agrees that sharks use a variety of markers to find their way, “but the point is [smell] participates in navigation," he says.
"Our study was only the first step in solving this mystery.”
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